1. Field of the Invention
The present invention relates generally to control valves and in particular to a pressure compensated spool valve for actuating a hydraulic cylinder which features a pressure compensator assembly for limiting the rate of movement of the cylinder and piston relative to each other under heavy loading conditions.
2. Description of the Prior Art
A double-acting hydraulic jack or lift cylinder is commonly used in a number of applications in which it is necessary to raise and lower heavy loads. Many types of valves are available for controlling the operations. One commonly available control valve is the five port shuttle valve for selectively applying high pressure hydraulic fluid from a supply line to either the head end or the rod end of a lift cylinder to produce extension or retraction, respectively. In this arrangement, the control valve includes a housing having a bore, a supply port for connection to a high pressure hydraulic source, rod and head cylinder ports, and a pair of return ports for connection to an exhaust tank. A spool is received in slidable engagement with the housing bore and is reciprocably movable through the bore from a neutral position blocking the supply port to first and second discharge positions opening the supply port for discharge through the bore into one of the rod and head cylinder ports. The control valve spool is normally provided with a mechanical centering spring to maintain the valve in the non-actuating position in which the supply port is blocked in the absence of a shifting force. Movement of the spool from the neutral or non-actuating position is provided by a pilot actuator, either hydraulically or pneumatically controlled. Shifting of the spool by the pilot actuator in one direction causes the supply port to be connected in communication with the head end of the cylinder when extension is desired, and shifting the spool in the opposite direction opens the supply port for discharge into the rod end of the cylinder when retraction is desired.
According to a common arrangement, the hydraulic jack or lift cylinder is provided with a piston/rod assembly which is fixed and a lift cylinder which is reciprocally movable with respect to the piston and rod. During the extension mode, the exhaust fluid from the rod end of the cylinder is discharged through the rod port into a tank exhaust return port and provides make-up fluid for pressurizing the head end. During the retraction mode, exhaust fluid from the head end is discharged through the head port and provides make-up fluid for pressurizing the rod end. The rate of movement of the lift cylinder relative to the rod and piston is proportional to the rate at which the cylinder is pressurized and the magnitude of the load being lifted.
A constant delivery pump is normally used for developing supply hydraulic pressure. Therefore in the lifting mode the rate of extension of the lift cylinder relative to the rod and piston for a given load is predictable and will gradually decrease as the load increases. However, as a load is being lowered, the rate of retraction of the lift cylinder relative to the rod and piston increases as the load increases. As the magnitude of the load carried by the cylinder continues to increase, the retraction rate may exceed safe limits and cause overheating of the hydraulic fluid or structural damage to the lift cylinder as the piston head collides with the lift cylinder, and also possibly causing damage to the load as the piston and cylinder suddenly collide in a runaway condition.
In the operation of a drilling rig in which a double acting hydraulic lift cylinder is used for lifting and lowering a drill string in a well bore, the load imposed on the hoist rig may range from a few thousand pounds to several thousand kips as additional lengths of tubing are connected into the drill string. Additionally, the drill bit and drilling collars together with other downhole equipment impose an additional substantial load on the drill string. Because the overriding concern to get the drill string into and out of the well as rapidly and safely as is economically possible, the lift cylinder is exposed to severe duty cycles. For example, the rig operator, in order to obtain maximum efficiency, will operate the hoist equipment at maximum speed. A crew member is apt to lower the drill string at free fall velocity to within a few feet of the stroke limit and then suddenly apply braking force to stop the drill string. Such an operation can impose extreme shock loads due to the deacceleration of the heavy pipe moving at high velocity. Although conventional masts are designed with a safety factor of three or four to withstand shock loading, as the length and weight of the drill string increases, the likelihood of a damaging shock load under free fall conditions substantially increases.
If the traveling block of the hoist rig is descending rapidly, the lift cylinder will be retracting as hydraulic fluid is introduced into the rod end of the cylinder. To suddenly stop the load, the operator will return the control valve to the neutral position, thereby blocking pressure flow to the cylinder and also blocking return flow which will have the effect of quickly deaccelerating the piston within the hydraulic cylinder. A relief valve is usually connected within the control system to prevent damage to the hydraulic system should the rig operator permit the load to fall at an uncontrolled rate. Although the relief valve will prevent damage to the hydraulic system, the derrick and other hoist equipment may be subjected to a substantial shock load, and there is a risk of a damaging impact which could shear the slips and cause the drill string to be dropped into the well bore should a sufficiently heavy drill string load be allowed to fall without control.